Crystalline Volume Ratio Research Articles

Carbon Heating Tube Used for Rapid Heating System

We report a microwave heating system with a carbon heating tube (CHT) which was made by a 4-mm diameter quartz tube filled with carbon particles and Ar gas at 1400 Pa. A 60-mm-long CHT was set in a cavity in which 2.45-GHz microwave was introduced. Via nearly complete absorption of a microwave power of 200 W by carbon in the CHT during multi-reflection of the microwave in the cavity, the CHT was effectively heated to 1279 °C at 33 s after the microwave initiation, which was observed by the radiation-type thermometer. Moreover, the control of the CHT temperature was demonstrated with a home-made proportional–integral–differential feedback circuit which regulated the magnetron power source using a signal of the thermometer. The control of temperature at 1100 °C was successfully realized. Crystallization of a 58-nm-thick amorphous silicon thin film formed on the glass substrate was demonstrated by mechanically moving the silicon sample just below the heated CHT. A high crystalline volume ratio of 0.92 was achieved. Furthermore, heating of 500- $\mu \text{m}$ -thick-n-type silicon substrate implanted with $1.0\times 10^{15}$ cm−2 phosphorus and boron atoms with the CHT resulted in activation of the doped regions. Rectified diode characteristics were achieved.

Preparation of Copper-Doped Nickel Oxide Thin Films by Sol-Gel Method Using Nickel and Copper Acetate

We have prepared nondoped and Cu-doped NiO thin films by sol-gel method on SiO2 substrate using nickel acetate and copper acetate as precursors. Thin films of NiO were formed by thermal annealing of gel films at 500 - 900°C for 10 h in air. X-ray diffraction (XRD) peaks of NiO have been observed at higher annealing temperature than 500°C, and intensity of the peaks increased with annealing temperature until 800°C. On the other hand, the dependency of XRD signal intensity on annealing temperature becomes less significant by Cu-doping. Nondoped NiO films show high transparency at lower annealing temperatures than 800°C. The result means that the film with high crystalline volume ratio is not consistent with the films with high transparency. Cu-doping decreases the transmittance of the films with low crystalline volume ratio. It is considered that the doped Cu atoms exist in amorphous fraction make the transmittance lower. Conductivity of the films is increased with Cu-doping only in the films annealed at 500 - 700°C. The highest conductivity obtained in the Cu-doped film is around 10-6 W-1cm-1 at 600°C. On the other hand, the Cu-doped films show similar conductivity to nondoped films at high annealing temperature, 800 or 900°C.

Crystallization of Silicon Thin Films By Microwave-Induced Rapid Heating

1.Introduction Crystallization of amorphous silicon (a-Si) thin films is important for fabricating thin film transistor (TFTs) and thin film solar cells [1]. Rapid heating is an attractive method with low thermal budget for fabricating these devices at a low cost. We have developed a simple heating method using a 2.45 GHz microwave associated with carbon powders [2,3]. Carbon powders are heated by microwave absorbance and samples are subsequently heated by the heat conduction. Temperature of carbon powders was increased above 1000 oC at 15 s and further increased to 1163 oC at 26 s by 1000 W microwave irradiation [3]. In this paper, we report crystallization of thin silicon films by the microwave heating. 2.Experimental procedure 2.4, 5.5, 10, 19, 50-nm-thick a-Si films were formed on 500-μm-thick 1.8x1.8-cm2-sized quartz substrates by plasma sputtering method. 100-nm-thick SiOxfilms were subsequently formed on the surface of 2.4-nm-thick a-Si film for preventing from evaporation of silicon during heat treatment. The samples were completely covered with carbon powders of 1 g. Then, microwave heating at 1000 W for 22 s was carried out using a commercial 2.45 GHz microwave oven. 3.Results and Discussion Analysis of optical reflectivity resulted in slightly decrease in film thickness by microwave heating from as-deposited 2.4, 5.5, 10, 19, and 50 nm to 1.8, 4.2, 6.0, 18, and 49 nm, respectively. Figure 1 shows Raman scattering spectra for the central points of the microwave heated silicon films with thicknesses between 1.8 and 49 nm as well as the initial 50-nm-thick amorphous silicon film. Analysis of the spectra was conducted with three Gaussian components of amorphous, nano-crystalline, and crystalline structures, respectively, to estimate the amorphous, nano-crystalline, and crystalline volume ratio [4]. Although, the initial a-Si film had a broad peak around 480 cm-1 associated only with the amorphous phonon mode, the 22-s-microwave-heated silicon films had sharp peaks associated with the crystalline silicon phonon mode appeared between 515 and 518 cm-1. The crystalline volume ratio was monotonously increased from 0.18 to 0.85 as film thickness increased from 1.8 to 49 nm. They also had broad tail spectra indicated by the arrows associated with the nano-crystalline phonon mode around 500 cm-1, whose intensity increased from 0.07 to 0.49 as the film thickness decreased from 49 to 1.8 nm. Large crystalline grain growth was limited by a thin film thickness. Moreover, photoluminescence which is attributable to the direct energy transitions in nano-crystalline silicon and small crystalline grains was observed for the microwave heated 1.8-nm-thick silicon film by 355 nm YAG laser irradiation. The present results demonstrate a capability of formation of high density of nano-crystalline particles by the microwave heating, which will be useful for application to fabrication of quantum-effect-nano-crystalline-thin-film soalr cells.

Annihilation of photo induced minority carrier caused by ion implantation and rapid thermal annealing

We report decreases in the minority carrier effective lifetime τeff of 700-µm-thick silicon substrates coated with 43-nm-thick thermally grown SiO2 layers by Ge ion implantation with a dose of cm−2 at 150 keV and rapid thermal annealing (RTA) at 1100 °C for 50 s. Ge ion implantation decreased the crystalline volume ratio in the top 157 nm region because of lattice damage. It also markedly decreased τeff to and s when 635 and 980 nm lights were continuously illuminated to the implanted surface, respectively. Small τeff values resulted from serious damage with a high surface recombination velocity S of cm/s and a short bulk lifetime of s in the 200 µm deep surface region. Although RTA successfully recrystallized the Ge-implanted surface region, it further decreased τeff to and s in the cases of 635 and 980 nm light illuminations to the top surfaces, respectively. RTA caused a high density of recombination sites with a high S value of cm/s on the top and rear surfaces. There was a high density of interface traps of eV−1 cm−2. Pa H2O vapor heat treatment at 260 °C for 3 h markedly increased τeff to and s in the cases of 635 and 980 nm light illuminations. The interface trap density was decreased to eV−1 cm−2. Low temperature post annealing is effective in curing the recombination defect states induced by RTA.

Crystallization of Amorphous Silicon Thin Films by Microwave Heating

ABSTRACTWe report crystallization of amorphous silicon (a-Si) thin films and improvement of thin film transistors (TFTs) characteristics using 2.45 GHz microwave heating assisted with carbon powders. Undoped 50-nm-thick a-Si films were formed on quartz substrates and heated by microwave irradiation for 2, 3, and 4 min. Raman scattering spectra revealed that the crystalline volume ratio increased to 0.42 for the 4-min heated sample. The dark and photo electrical conductivities measured by Air mass 1.5 at 100 mW/cm2 were 2.6x10-6 and 5.2x10-6 S/cm in the case of 4-min microwave heating followed by 1.3x106-Pa-H2O vapor heat treatment at 260°C for 3 h. N channel polycrystalline silicon TFTs characteristics were improved by the combination of microwave heating with high-pressure H2O vapor heat treatment. The threshold voltage decreased from 5.3 to 4.2 V and the effective carrier mobility increased from 18 to 25 cm2/Vs.

Nano-Crystalline Silicon-Based Bottom Gate Thin-Film Transistor Grown by LTPECVD With Hydrogen-Free He Diluted ${\hbox{SiH}} _{4}$

The bottom-gate nc-Si based thin-film-transistors (TFTs) grown by using the low-temperature plasma-enhanced chemical vapor deposition (LT-PECVD) system with He diluted SiH are demonstrated. With the RF plasma power increasing from 20 to 100 W, the crystalline volume ratio of the nc-Si inside the a-Si:H film significantly increases from 12.5% to 32%, and its deposition rate is also enhanced from 9.5 to 14.5 nm/min. The faster deposition at higher plasma greatly suppresses the residual oxygen content in nc-Si film to 4% or less, which reduces the flat-band shifted voltage of the MOS diode by 2 volts. The increased crystalline volume with suppressed oxide in nc-Si films contribute to the enhanced Hall mobility and conductivity. The nc:Si TFT decreases its threshold voltage from 3.3 V to 2.7 V, and enlarges its field mobility from 0.3 to 1.3 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> V-s. The defect density in the nc-Si TFTs further decrease by one order of magnitude to 7.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> eV, which causes a shrinkage on the sub-threshold operation range to make easier the operation of the nc-Si TFTs entering into the above-threshold regime at lower voltage. The hydrogen-free He diluted SiH growth has shown its compatibility with the conventional recipe for the high-mobility nc-Si TFT fabrication.

Infrared semiconductor laser irradiation used for crystallization of silicon thin films

We report the rapid thermal crystallization of silicon films using infrared semiconductor laser. Carbon films were used on silicon films to absorb the laser light. Uniform crystalline regions were achieved by a line shape laser beam with a length of 20μm. Polycrystalline silicon thin film transistors were fabricated in crystallized regions. The effective electron carrier mobility and threshold voltage were achieved to be 130cm2/Vs and 0.4V, respectively, when the crystalline volume ratio of the silicon films was 0.95.

Crystallization of sputtered amorphous silicon induced by silver–copper alloy with high crystalline volumeratio

Silver–copper alloy-induced crystallization of sputtered a-Si has been studied. In this alloy, Cu acts as a catalyst to accelerate the crystallization, while Ag acts as a new kind of buffer layer, different from Al 2O 3 and Si 3N 4, to obtain well-crystallized poly-Si films with short annealing time and free of post-treatment for ohmic contact. When the Cu content is limited to below 30%, Ag can effectively slow down the diffusion of Cu into Si and decrease the Cu–silicide nuclei density to improve the crystalline volume ratio from 80% to over 90%. A 1:4 ratio of Cu to Ag yields the best result. The crystalline volume ratio and Hall mobility reach nearly 100% and 29.4 cm 2/V s, respectively. This high quality poly-Si film demonstrates a promising application in solarcells.

Recrystallization Behavior of Silicon Implanted with Phosphorus Atoms by Infrared Semiconductor Laser Annealing

We report the recrystallization behavior of silicon implanted with phosphorus atoms at 10 and 70 keV with a dose of 2×1015 cm-2. The samples were coated with a 200-nm-thick diamond-like-carbon optical absorption layer and then annealed by irradiation with a 940 nm continuous-wave laser at 70 kW/cm2. An analysis of optical reflectivity spectra showed an in-depth distribution of the crystalline volume ratio. The amorphized surface regions produced by the phosphorus implantation were recrystallized from the bottom region by laser annealing. They were almost completely recrystallized by the laser annealing for 2.6 ms. The in-depth profiles of phosphorus concentration hardly changed for the laser annealing for 2.6 ms. The implanted regions were effectively activated by the laser annealing and the sheet resistance markedly decreased to 102 and 46 Ω/sq for implantion at 10 and 70 keV, respectively.

The influence of SiN x substrate on crystallinity of μc-Si film used in thin film transistors

This paper found that the crystalline volume ratio (Xc) of μc-Si deposited on SiNx substrate is higher than that on 7059 glass. At the same silane concentration (SC) (for example, at SC=2%), the Xc of μc-Si deposited on SiNx is more than 64%, but just 44% if deposited on Conning 7059. It considered that the `hills' on SiNx substrate would promote the crystalline growth of μc-Si thin film, which has been confirmed by atomic force microscope (AFM) observation. Comparing several thin film transistor (TFT) samples whose active-layer were deposited under various SC, this paper found that the appropriate SC for the μc-Si thin film used in TFT as active layer should be more than 2% and Xc should be around 50%. Additionally, the stability comparison of μc-Si TFT and a-Si TFT is shown in this paper.

Laser Crystallization of Silicon Thin Films Using Photo Absorption Layer Formed by Spin Coating of Carbon Particles

ABSTRACTWe report crystallization of amorphous silicon films 50∼1400 nm thick formed on glass substrates by irradiation of an infrared semiconductor laser. Carbon particles with a mean diameter of 200 nm were uniformly coated on the silicon films as the optical absorption layer. Samples were irradiated with a 940-nm continuous wave (CW) semiconductor laser with a power of 20 W. A high optical absorbance 76.5% at 940 nm of the carbon particles layers resulted in annealing of silicon films by heat diffusion from the carbon particle layers. After removing the carbon particles layer, Raman scattering spectral measurements were carried out. A high scattering intensity and a sharp crystalline silicon phonon band were observed. The analysis of Raman spectra revealed a crystalline volume ratio almost 1.0. The analysis of the optical reflectivity spectra also revealed that the silicon films were crystallized in the whole thickness by the present method.

Heating Layer of Diamond-Like Carbon Films Used for Crystallization of Silicon Films

Rapid thermal crystallization of silicon films with a heating layer of diamond-like carbon (DLC) films is reported. DLC films 200 nm thick have an optical absorbance higher that 0.7 for wavelengths shorter than 1000 nm. They are also heat resistant to about 5000 K. A crystalline volume ratio of 0.85 is achieved for silicon films through heat diffusion from DLC heated by 30-ns-pulsed XeCl excimer laser irradiation at 200 mJ/cm2 for 100 nm DLC/5 nm SiO2/25 nm Si/quartz, while it is only 0.4 for 25 nm Si/quartz because of high reflection loss. Crystallization of silicon films is also achieved by 1064 nm YAG laser heating of the 200 nm DLC layer overlying the silicon films.

Pulsed laser crystallization of silicon–germanium films

Pulsed-XeCl excimer laser crystallization of germanium (Ge) and silicon–germanium (SiGe) alloy films formed on quartz glass substrates was investigated. The transient conductance measurements reveled that germanium films were rapidly melted and solidified because of low latent heat compared to silicon films. Formation of silicon–germanium alloy was also observed in the case of laser annealing the Ge/Si layered structure. The crystalline volume ratio was estimated almost 1.0 for Si 0.4Ge 0.6 films because of small disordered electronic states at grain boundaries, while it was 0.85 for Si films.

Electrical and structural properties of poly-SiGe film formed by pulsed-laser annealing

Electrical and structural properties of polycrystalline silicon germanium (poly-SiGe) films fabricated by pulsed-laser annealing were investigated. Observation of laser-induced melt-regrowth of SiGe films using transient conductance measurement revealed that the melt depth and the crystallization velocity increased as Ge concentration increased. The increase of the crystallization velocity resulted in increase of the average size of crystalline grains from 66 to 120 nm at the laser energy density of 360 mJ/cm2 with increasing Ge concentration from 0 to 60%. The crystalline volume ratio obtained by reflectivity spectra in the ultraviolet region also increased from 0.83 to 1.0. Numerical analysis revealed that the density of electrically active defects decreased from 3.5×1018 to 1.1×1018 cm−3 as Ge concentration increased from 0 to 80%. The density of defect states of Si0.8Ge0.2 films were reduced from 3.5×1018 to 1.9×1018 cm−3 by 13.56-MHz hydrogen plasma treatment at 250 °C, 30 W, and 130 Pa for 30 s. However, the plasma treatment did not reduce the defect density for Si0.4Ge0.6 and Si0.2Ge0.8 films.

The effect of thermal annealing on crystallization in a-Si:H/SiO2multilayers by using layer by layer plasma oxidation

Two post-treatments consisting of rapid thermal annealing (RTA) and furnace annealing(FA) were used to crystallize a-Si:H sublayers in a-Si:H/SiO2multilayers fabricated by alternate plasma-enhanced chemical vapour depositionof a-Si:H layers and plasma oxidation. Raman measurements show thatthe crystallization process of a-Si:H sublayers strongly depends on thepost-treatment process. RTA is advantageous for nucleation, in which thesize of the nuclei increases with higher annealing temperatures, and thecrystalline volume ratio increases with longer annealing times. In FA,however, higher temperatures are required for further crystallization such asincreasing the grain size and crystalline volume ratio. The ultimate size ofnc-Si grains can be accurately determined by the thickness of the a-Si:Hsublayer. Moreover, the mechanism of the constrained crystallization willalso be discussed in accordance with the thermodynamic theory.

Epitaxial growth of polycrystalline films formed by microwave plasma chemical vapor deposition at low temperatures

We report epitaxial growth of polycrystalline silicon films using microwave-induced PECVD from initial laser crystallized silicon formed on glass substrates. Undoped silicon was first crystallized by a method of pulsed laser-induced rapid melt-regrowth. Crystalline volume ratio of 100 nm thick microcrystalline silicon layer subsequently deposited on the bottom laser crystallized layer increased from 0.2 to 0.37 as the ratio of the bottom layer increased from 0.69 to 0.8. Epitaxial growth ratio was determined as 0.45 for the present CVD method. The electrical conductivity of doped microcrystalline silicon top layer also increased because of increase crystalline volume ratio.